The present invention relates to an optoelectronic gas sensor based on optodes as well as an electronic component for producing such an optoelectronic gas sensor.
An optoelectronic gas sensor is described in a technical article xe2x80x9cA field hardened optical waveguide hybrid integrated-circuit, multi-sensor chemical probe and its chemistryxe2x80x9d by Richard J. Polina et al. in SPIE, vol. 3105, pages 71-78. This known gas sensor based on optodes is diagramed schematically in FIG. 6, and its properties are described briefly below.
From an LED 33, a light bundle is divided vertically by two mirrors 35, 36 into two parts L1, L2 and reflected laterally, so it goes to a measuring segment 38 made of optode material and a reference segment 39. Light beam L2 passing through optode segment 38 is in turn reflected down vertically on a mirror surface 37, so it reaches the photosensitive surface of a first photodiode 32, while light beam L1 passing through reference segment 39 is reflected down vertically onto the photosensitive surface of another photodiode 31 at another mirror surface 34. Optode segment 38 and reference segment 39, mirror surfaces 34-37 and photodiodes 31 and 32 are arranged symmetrically about LED 33 which is arranged at the center. The optode material of measuring segment 38 is exposed opposite the gas to be measured (arrow), so this gas has access through an opening provided in the chip casing (not shown).
FIG. 7 shows schematically a gas measuring sensor 30 equipped with three successive sensor units 301, 302 and 303 according to FIG. 6.
Due to the method of coupling and reflection of light bundles L1 and L2 and mirror surfaces 34-37, first from the vertical into the lateral direction and then from the lateral back into the vertical direction, the known gas sensor chip shown in FIG. 6 and described in the technical article cited above is relatively long and broad (e.g., 3 cm long and 0.35 cm broad), so a gas measuring sensor 30 according to FIG. 7 constructed using multiple gas sensor chips 301-303 arranged in successive rows turns out to be rather long. In addition, such a known gas sensor chip and gas measuring sensor 30 implemented with it is relatively expensive. Furthermore, various aging phenomena on separate chips 301-303 can lead to unwanted measurement errors.
An object of the present invention is to make possible an improved optoelectronic gas sensor based on optodes, so that it will be less expensive and will have much smaller dimensions, and so that an electronic component according to the present invention can be made available for manufacturing an optoelectronic gas sensor without requiring additional optical components such as mirrors and prisms.
According to a first embodiment of the present invention, the object is achieved by providing an optoelectronic gas sensor based on optodes, where multiple separate photosensitive elements and an opto-transmitter located centrally between them are integrated into or onto a semiconductor substrate; this is characterized in that the photosensitive elements lie in one plane in the substrate, and together with a lateral emission area of the opto-transmitter emitting light laterally they are covered by sections of the optode material whose thickness and refractive index are selected so that light emitted laterally from the emission area is guided to the photosensitive elements by total reflection in the optode material in each transmission branch.
According to a second embodiment of the present invention, an optoelectronic gas sensor based on optodes achieving the above object is made available, where multiple separate photosensitive elements and an opto-transmitter located centrally between them are integrated into or onto a semiconductor substrate; this is characterized in that the photosensitive elements lie in one plane in the substrate and are each covered by a section of the optode material, the opto-transmitter is spaced a distance away from the sections of the optode material through an annular gap, and the thickness of the optode material is much smaller than the height of the opto-transmitter, so the light emitted by the opto-transmitter is emitted into air and then reaches the photosensitive elements through the optode material either directly or after being reflected on the inside walls of a casing surrounding the gas sensor chip.
One of the photosensitive elements and the layer covering it may form a reference segment. The optode material of the measuring segments is made of a gas-sensitive polymer carrier material to which is added at least one indicator substance from the group of compounds including, for example, azobenzenes, acetophenones, corrins, porphyrins, phthalocyanines, macrolides, porphyrinogens, nonactin, valinomycin and/or complexes thereof with transition metals of secondary groups I-II and V-VIII. However, the layer covering the reference segment may be made of a polymer carrier material without any added indicator substance.
In an embodiment of a layout according to the present invention, the photosensitive elements of the optoelectronic gas sensor with the sections of the optode material covering them or with the polymer carrier layer covering the reference segment may be arranged in sectors with central symmetry around the opto-transmitter. For example, in this way four symmetrical transmission branches may be formed, including three sensor segments and one reference segment.
The chip forming the optoelectronic gas sensor may be designed to be square, pentagonal, hexagonal, heptagonal or octagonal or even circular, for example. Of course, such an optoelectronic gas sensor may also include less than or more than four transmission branches.
With an optoelectronic gas sensor implemented according to the first embodiment, the individual transmission branches are separated by barriers, so that the individual transmission branches are not influenced optically by the stray light coming from the optode material. The height of these barriers may be selected to be approximately the same as the height of the central photosensor. In addition, all locations on the chip that are not photosensitive may be mirrorized if necessary, likewise the side walls of the barriers.
The substrate of the chip may be made of n-type silicon, and the photosensitive elements may be made of p-type regions of silicon integrated into the n-type silicon substrate. In this way, the photosensitive elements form photodiodes. The opto-transmitter is preferably an LED, but multiple LEDs may also be used to define the wavelength.
The thickness of the optode material over the photosensitive elements may be 200 xcexcm to 300 xcexcm and preferably in the range of 220 xcexcm to 260 xcexcm.
With an optoelectronic gas sensor constructed according to the second embodiment of the present invention, the thickness of the optode material is much less than the height of the LED and amounts to approx. 5 xcexcm to 20 xcexcm, preferably 5 xcexcm to 10 xcexcm, while the height of the opto-transmitter (LED) is much greater, and may amount to approx. 300 xcexcm.
To produce an optoelectronic gas sensor, an electronic component provided for this purpose may be designed so that multiple separate photosensitive elements are integrated into or onto a semiconductor substrate in sectors with central symmetry while maintaining a certain mutual spacing; a thin dielectric insulation layer covers all the photosensitive semiconductor areas; contact openings are provided with contacts to the photosensitive semiconductor elements at defined peripheral locations on the photosensitive semiconductor elements; and metallized strips are provided in the spaces between the photosensitive elements leading to a central contact pad for connecting the LED functioning as the opto-transmitter.
In this way, four equally large photodiodes having a common cathode formed by the substrate and an area of the individual photosensitive areas of 0.8 to 1 mm2 may be integrated into in a chip having an edge length in the range of approx. 200 xcexcm to 300 xcexcm, with the total chip height being approx. 400 xcexcm to 500 xcexcm, in an electronic component suitable for production of an optoelectronic gas sensor.
For practical use, the chip of the optoelectronic gas sensor is mounted in a casing (preferably SMD) and protected by a cover. This cover has openings over the locations coated with gas-sensitive material so that gas can penetrate.
With respect to good processability, it is possible to mount the casing cover on a circuit board before applying the gas-sensitive materials and to solder the electronic component which later forms the optoelectronic gas sensor into a corresponding electronic circuit without coating it. Therefore, the gas-sensitive materials cannot be destroyed or damaged in any way by the heat of the soldering operation. The openings in the casing cover through which the coating is subsequently applied or through which the gas enters in subsequent use can be sealed with an adhesive film, for example, in the soldering operation to prevent flux from entering.
The present invention offers at least the following advantages, in particular, in comparison with conventional optoelectronic gas sensor based on optodes:
This sensor has extremely small geometric dimensions due to the integration of all function units of the sensor (i.e., the electronic components and the optode paths on a silicon chip).
According to the first embodiment of the present invention, the optode segments, i.e., the gas-sensitive polymer and the reference segment, can assume the function of the passive optical system otherwise necessary, such as prisms or mirrors in guiding the light from the opto-transmitter to the optode and from the optode to the photodiode, because of the small distance between the opto-transmitter (LED) and the photosensitive elements (photodiodes) This guarantees that the atmosphere surrounding the gas can act on a large surface area of the optode.
The small distance between the LEDs and the photodiode causes a high efficacy in coupling light between these two components. This means a low power consumption.
The adjusted emission characteristics of the LEDs laterally, achieved by mirrorizing the top and bottom sides of them, enhances this effect.
coupling losses are extremely low, i.e., no additional passive optical system is necessary, due to direct coupling of light from the LEDs to the optode segments and the reference segment and from there to the photodiodes.
The barriers guarantee low crosstalk.
The reference and measuring branches have a high symmetry because the photodiodes are monolithically integrated.
The optoelectronic gas sensor implemented according to the second embodiment of the present invention has an advantage in comparison with that described above in that the optode layer and the layer of the reference segment can be much thinner. These layers have a thickness of approx. 5-10 xcexcm. They do not function primarily as light guides because the light is first emitted into air by the LED and then passes through the optode layers and/or the reference layer to the photodiodes either directly or after being reflected by the surrounding casing. The barriers have only a subordinate importance here.